(1) Field of the Invention
The present invention relates to a metal hydride storage cell and a method of producing a hydrogen absorbing alloy electrode using a hydrogen absorbing alloy which electrochemically absorbs and releases hydrogen. The present invention specifically relates to an improved method of producing a sintered hydrogen absorbing alloy electrode.
(2) Related Art
As technologies in the electronics field have developed in a rapid pace, portable or cordless electronic devices have been brought into markets. There are increasing demands for compact, light secondary cells having high energy density and high performance efficiency for use in such electronic devices as the power source.
Under such circumstances, metal hydride storage cells including a hydrogen absorbing alloy negative electrode are drawing attention. This is because such metal hydride storage cells are able to have higher energy density than nickel-cadmium storage cells or lead storage cells, and therefore, are suitable for high-capacity cells. Moreover, they are clean power sources.
In the metal hydride storage cells, oxygen gas is generated from the positive electrode when the positive electrode is overcharged. Therefore, a capacity greater than that of the positive electrode is set as the capacity of the negative electrode so that the generated oxygen gas is consumed in the negative electrode.
Hydrogen absorbing alloy electrodes are generally produced as follows.
First, a slurry is made by mixing hydrogen absorbing alloy powder with an organic binding agent and water. Then, the slurry is applied onto or filled into a core plate and is then dried. As the hydrogen absorbing alloy, a rare-earth-nickel alloy, a magnesium-nickel alloy, or a titan-nickel alloy is used.
However, hydrogen absorbing alloy electrodes produced with the above method tend to have low electronic conduction and show poor charge/discharge characteristics in high-rate charge/discharge and in low temperature since the binding agent is included in between the hydrogen absorbing alloy powders.
The hydrogen absorbing alloy electrodes produced with the above method also have a problem that the hydrogen absorbing alloy is gradually oxidized and the capacity of the negative electrode is reduced as the negative electrode consumes the oxygen gas generated in the positive electrode due to an overcharge. This shortens the period of charge/discharge cycles.
To solve the above problems, a method has been proposed in which a non-sintered electrode is changed to a sintered electrode.
For example, Japanese Laid-Open Patent Application No.5-258750 discloses a method in which a hydrogen absorbing alloy electrode is produced by baking a hydrogen absorbing alloy powder with nickel attached to its surface for a short period at 400-1000.degree. C. for sintering. With this method, a nickel-rich alloy layer is formed on the surface of the hydrogen absorbing alloy. This increases the catalysis efficiency of the surface without increasing the hydrogen equilibrium pressure of the hydrogen absorbing alloy, and improves the electrochemical hydrogen absorption speed and the charge/discharge efficiency in charging/discharging.
However, the electrode produced with the above method does not have enough strength to securely retain the hydrogen absorbing alloy around the core plate. As a result, the hydrogen absorbing alloy often separates from the core plate during cell construction.
The separation between the hydrogen absorbing alloy and the core plate may be prevented by adding a binding agent after the electrode is sintered. In this case, however, the added binding agent often disturbs electrochemical reaction.
Japanese Laid-Open Patent Application No.1-130467 discloses a method in which a hydrogen absorbing alloy powder is given an acid treatment or an alkali treatment so that a nickel-rich layer, which, as a catalyst, reduces oxygen, is formed on the surface of the hydrogen absorbing alloy powder. An electrode plate is then formed by using this hydrogen absorbing alloy powder. The electrode plate is baked to be sintered to increase the electronic conductivity of the electrode plate, and to improve charge/discharge characteristics in high-rate charge/discharge and in low temperature.
However, it is required in this method to bake the electrode plate at a high temperature of 1000.degree. C. for sintering. Furthermore, this method has the same problem as the earlier method. That is, the electrode produced with the above method does not have enough strength to securely retain the hydrogen absorbing alloy around the core plate. As a result, the hydrogen absorbing alloy often separates from the core plate during cell construction.
Japanese Laid-Open Patent Application No.5-62676 discloses a method in which a micro powder of Ni or Co is added as a sintering auxiliary material to a mixture of an MmNi.sub.5 hydrogen absorbing alloy powder and an Mg.sub.2 Ni hydrogen absorbing alloy powder. An electrode plate is formed by using this mixed powder. The electrode plate is then baked to be sintered to increase the electronic conductivity of the electrode plate, and to improve charge/discharge characteristics in high-rate charge/discharge and in low temperature.
However, the metal hydride storage cell produced with the above method has problems that a corrosion of the hydrogen absorbing alloy tends to grow fast and that the cycle life is short.
The above application document also discloses that a hydrogen absorbing alloy powder whose surface has been given an alkali treatment may also be used in the method, providing the same effect. However, when a hydrogen absorbing alloy powder is given an alkali treatment, a hydroxide is formed on the surface of the powder. The hydroxide decomposes when the hydrogen absorbing alloy powder is baked for sintering. This causes the hydrogen absorbing alloy powder to be oxidized, which tends to reduce the capacity of the hydrogen absorbing alloy electrode.